I'm wondering if the excessive oil consumption and the deposits on #3 and #4 plugs are related? Because 3+4 share a carb I jumped to the conclusion that it was carb related. But I can't see anything obvious.

Is there an pressurised oilway from the block to the head between #3 and #4? I'm wondering if I have another head gasket issue?

It's possible that an over rich mixture on the rear carb has allowed the rear 2 cylinders to become "bore washed", ie too much petrol has washed away the coke lips in the cylinders and the coke buildup on the pistons, which would result in increased oil consumption as every engine relies partially on this coke buildup to seal the pistons.
Long term, bore washing can also cause accelerated wear in the piston rings and bores, petrol being a lot worse lubricant than oil. So you may have ring damage from bore washing, or the rings may just be a bit tired anyway. The former would fit the symptoms best, but i'm guessing that a camera down the bores is a first step and my money is on a set of rings as a minimum. Sorry to be the harbinger of doom, but that's how I see it!

It's possible that an over rich mixture on the rear carb has allowed the rear 2 cylinders to become "bore washed", ie too much petrol has washed away the coke lips in the cylinders and the coke buildup on the pistons, which would result in increased oil consumption as every engine relies partially on this coke buildup to seal the pistons.
Long term, bore washing can also cause accelerated wear in the piston rings and bores, petrol being a lot worse lubricant than oil. So you may have ring damage from bore washing, or the rings may just be a bit tired anyway. The former would fit the symptoms best, but i'm guessing that a camera down the bores is a first step and my money is on a set of rings as a minimum. Sorry to be the harbinger of doom, but that's how I see it!

Steve

Would a compression test show worn rings? If they are worn shouldn't the compression be reduced?

With a compression test and you have some reading you do know that there is a piston inside going up and down and a few valves are openening and closing. It will tell you nothing about oil consumption.

With a compression test and you have some reading you do know that there is a piston inside going up and down and a few valves are openening and closing. It will tell you nothing about oil consumption.

A good suggestion to check the bores has already be given.

Jeroen.

I meant as a test for worn piston rings. Surely if the rings are worn, you would get reduced compression on that cylinder. If oil can get passed the rings, then how can they hold compression in the cylinder?

You can have rings in a good condition that have the gap ok and do seal compression wise but when you have nice and shiny walls you probably have an oil burner. A slightly leaky valve can also cause oil consumption. You can't tell by a compression test.

As I said the only diagnosis you can have with an compression test is that you actually have pistons and valves in your engine. To diagnose oil consumption or rings or valves you need other methods to test. A cilinder leak test for example shows the actual wear/leak and you can determine if it's rings or valves or whatever.
With an endoscoop you visually can check the walls if there's a honing pattern left or not.

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